It has long been known that turning any vehicle at high speed in an uncoordinated fashion is uncomfortable for passengers and may even cause passengers or cargo to move in one direction or the other, either in response to centrifugal force or gravity. For instance, if an aircraft takes a sharp level turn by rotation in its yaw axis, the resultant force on passengers and cargo is essentially a sideways force radially outward from the center of the turn. On the other hand, if the aircraft makes a sharp bank angle and a nose up turn, with no rotation in the yaw axis, then the gravitational force will no longer be normal to the floor of the aircraft, and can literally slide passengers off the sides of their seats. A coordinated turn is one in which the net acceleration is in a direction normal to the floor of the aircraft. This is achieved by combining the desired amount of roll with a correct amount of yaw, as is well known in the art. Examples of systems that provide coordinated turns are U.S. Pat. Nos. 4,003,532, 4,067,517 and 4,206,891.
It is common at low airspeeds to utilize yaw to control the heading of the aircraft; but at high speeds, it is common to employ roll to effect a turn, utilizing yaw only to coordinate the turn. When entering a coordinated turn, a pilot may, for instance, place his feet on pedal switches on the yaw-controlling pedals of the aircraft. As the aircraft rolls over from a wings level condition, roll rate is utilized to provide a yaw input to tend to coordinate the turn initially. In the steady state, integrated lateral acceleration is utilized to provide the yaw input to coordinate the turn. In a turn of a constant speed and roll angle, once the lateral acceleration integrator output has reached the correct amount, the lateral acceleration becomes zero (since there is no sideways movement in a perfectly coordinated turn).
When the pilot is controlling a turn by lateral movement of the cyclic pitch stick to induce the desired degree of roll, and places his feet on pedal switches so as to cause an automatic yaw trim system to provide coordination for the turn, if the yaw trim system should fail, the pilot will almost naturally take over coordinating the turn by means of the foot pedals, upon sensing sideslip (lateral acceleration). Similarly, if a flight director is steering the aircraft in roll, and an automatic yaw trim system is providing automatic coordinated turn in response to the roll induced by the flight director, if there is a failure in the yaw channel, and the yaw channel is automatically shut down, there is no need to disengage the roll channel since the turn will simply become increasingly uncoordinated if the roll angle varies, particularly at the end of the turn (a self-correcting situation). Therefore, there is no need to shut down an automatic roll channel as a consequence of a shutdown of the yaw trim system. On the other hand, if a flight director is providing automatic steering of the aircraft by inducing desired roll angles, and a yaw trim system is providing automatic trim coordination in response to roll rate and lateral acceleration induced by the roll angles, if there is a hard-over failure of the roll channel inner loop and the roll outer loop channel is consequently shut down, the inner loop can cause continuous roll input commands, which cause continuous increase in roll angle. The coordination of the turn masks this from the pilot, so he may not react in a timely fashion. And, increasing roll angle with commensurate yaw coordination may induce a spiral maneuver, which could be dangerous.
Another problem with automatic flight control systems is that the addition of every function provides the need for more switches, indicators and warnings in the cockpit of an aircraft. For instance, automatic shutdown systems must be provided with indicators so the pilot will clearly know that further automatic control in one or more axes of the aircraft will not be forthcoming, and recover control of the aircraft himself. However, constraints of cost, weight and cockpit complexity renders it desirable to provide minimum number of system turn on switches and fault indicators consistent with safety.